Caption: Recombinant virus research. Researcher holds a vial of genetically-engineered (recombinant) viruses in a high-security laboratory. Viruses can be used as vectors for introducing new genes into cells. Once a virus infects a cell it inserts its own genes (DNA) into the cell's genetic material, causing it to produce more viruses. The virus can be genetically-engineered so that when it infects a cell, it inserts DNA that causes the cell to produce a specific chemical. Bacteria may be infected to produce novel chemicals, whilst the virus may be used on humans (gene therapy) to make their cells produce an essential chemical & so cure a disease.